In a standard package--for example, as described in U.S. Pat. No. 4,774,635 or as illustrated in FIG. 1--an electronic component such as a semiconductor integrated circuit chip 15 (FIG. 1) is electrically connected to external circuitry by means of electrically conductive wires (or "wire segments") 13 and electrically conductive fingers 11. These conductive fingers 11 originally are formed by patterning, as by etching or stamping, a typically rectangular metal sheet or strip of metal ("lead frame" or simple "frame"), typically comprising copper, in its vertical Z (thickness) direction in accordance with a prescribed pattern in the horizontal XY plane of a major surface of the frame. Hence, the term "lead frame fingers" can be applied to the conductive fingers 11. The metal frame initially has a relatively thick peripheral portion running along its perimeter. Typically, the conductive fingers 11 (initially not bent) radiate inward either from two opposing sides of from all four sides of the frame toward a central region thereof where the chip ("die") 15 is to be located. Also radiating inward from each of the four corners of the perimeter of the frame is a finger, to be referred to as an "auxiliary support finger" or simply "auxiliary finger" (not shown). More particularly, each such auxiliary finger radiates inwardly typically to a respective corner of a square or rectangular metal plate 12, to be referred to as a "mounting pad" (also known as a "die paddle" or simply a "paddle").
The mounting pad 12 initially is part of a central portion of the frame and hence initially is integral with the conductive fingers 11, i.e., prior to the patterning, as by the etching or stamping. The mounting pad 112 is also integral with the auxiliary fingers. The mounting pad 12, being made of (thermally conducting) metal, thus advantageously facilitates heat dissipation from the chip 15 to the surrounding environment.
The mounting pad 12, the conductive fingers 11, and the auxiliary fingers are all simultaneously formed by the aforementioned patterning of the frame. During this patterning, an inner portion 11.1 of each conductive finger 11--i.e., the portion of each conductive finger 11 located closest to the mounting pad 12--is removed, so that a prescribed horizontal distance L separates each conductive finger 11 from the mounting 12. On the other hand, the auxiliary fingers remain integral with the pad 12. These auxiliary fingers thus supply mechanical support for the mounting pad 12 until after the encapsulation layer 14 has been formed, as described below.
The auxiliary fingers are then bent by a precision bending step, typically using a standard forming tool, in such a way as to provide a prescribed "downset" D for the die paddle 12--i.e., a distance D between the top surface of the die paddle 12 and the top surface of the inner ("internal") portion 11.1 of each of the conductive fingers 11. Optionally, the conductive fingers are, together with the mounting pad and auxiliary fingers, then plated with a precious metal.
The bottom major surface of the chip 15 is then bonded to the mounting pad 12, typically by an epoxy adhesive; and electrical contact pads 17, located on the top major surface of the chip 15, are electrically coupled to the conductive fingers 11 via the electrically conductive wires 13. The assembly comprising the conductive wires 13, the mounting pad 12, the chip 15, and the internal portion 11.1 of the conductive fingers 11 ("internal fingers 11.1") are then enclosed, with the aid of a suitable metallic molding tool, in the encapsulation layer 14, such as an epoxy or plastic molding compound. The conductive fingers 11 are then severed from the thick peripheral portion of the frame at a prescribed position on the thin portion of the frame located between the thick peripheral portion and the boundary of the encapsulation layer 14. The resulting severed conductive fingers 11 thus have outer portions ("external fingers") 11.2 of desired lengths. The auxiliary fingers (not shown), however, are severed typically flush with the boundary of the encapsulation layer 14, so that virtually no external portion of these auxiliary fingers remains, but only auxiliary finger remnants (not shown) remain.
The external fingers 11.2 are then shaped, typically by bending, whereby their bottom surfaces fall below the bottom surface of the encapsulation layer 14. In this way the electrical contact pads 17 located inside the package 10 can be electrically connected through the conductive fingers 11 to the external circuitry, such as a second level interconnection board, which is typically a printed wiring board or printed circuit board. The amount of this bending is selected to ensure proper contact of the external fingers 11.2 to the printed wiring (or printed circuit) board and to leave sufficient space for cleaning solution to penetrate between the underside of the package and the top surface of the board. The thus severed conductive fingers 11 therefore constitute the I/O (Input/Output) conductive leads for the circuitry of the semiconductor chip 15.
The above-mentioned downset distance D of die paddle support fingers is desirable to ensure that the wires 13 are set at the proper level within the thickness of the encapsulation layer 14--i.e., so that the top of these (wires) wire segments do not protrude outside of the top surface of the encapsulation layer 14.
This packaging scheme results in a package 10 in which the thickness of the paddle 12 is equal to the mutually equal thicknesses of the conductive fingers 11 and the auxiliary support fingers, since they were all derived, by patterned etching or stamping, from the same frame in regions thereof where it had a uniform thickness. In particular, this uniform thickness typically is equal to approximately 0.15 mm, in order to assure sufficient electrical conductivity, as well as to assure sufficient mechanical strength and robustness of the conductive fingers 11 especially in the finished package, and to assure sufficient mechanical strength and robustness of the auxiliary support fingers during fabrication of the package. This equality of the thicknesses of the paddle 12 and of the conductive fingers 11 imposes an undesirable limitation on how much the thickness of the package 10--i.e., the thickness of the encapsulation layer 14--can be reduced.
In order to avoid this limitation on how much the thickness of the package can be reduced, Japanese Kokai Patent Application No. Hei 1[1989] 1-151259, published on Jun. 14, 1989, teaches a thinner package by utilizing a thin mounting pad--that is, a mounting pad which has been made (by patterning) from the same frame as that from which the conductive fingers have been made (by patterning), but which has been made thinner than the conductive fingers by means of etching a selected portion of one of the major surfaces of the frame prior to attaching the chip to the resulting thinner portion of the frame, i.e., the portion that then serves as the mounting pad. However, such etching involves a time-consuming, complex, and costly two-sided masking procedure, as well as a procedure in which control over the resulting thickness is difficult. Moreover, the etched surfaces are not as smooth as is desired, particularly for bonding the bottom major surface of the chip to the top major surface of the mounting pad.
Therefore, it would be desirable to have a method of making a thinner package utilizing a mounting pad that is thinner than the conductive fingers but is not made to be thinner by means of an etching procedure.